This invention relates generally to an electrolytic process for producing chloride free alkali metal nitrates. More particularly, it relates to a process for producing a mixture consisting essentially of sodium and potassium nitrates in a membrane cell employing particular permselective membranes and operating parameters.
Both sodium and potassium nitrates are used in a wide variety of industrial and agricultural uses. While the world needs for NaNO.sub.3 are largely met from naturally occurring Chile saltpeter, there are few natural sources of KNO.sub.3. As a result, the largest part of currently used KNO.sub.3 is made synthetically by the reaction: EQU KCl+NaNO.sub.3 .fwdarw.KNO.sub.3 +NaCl
usually starting from natural ores containing KCl (e.g. silvinite--NaCl.KCl or carnallite--KCl.MgCl.sub.2.6H.sub.2 O) or K.sub.2 SO.sub.4 (langbeinite--K.sub.2 SO.sub.4.2MgSO.sub.4 and kainite--K.sub.2 SO.sub.4.MgSO.sub.4.3H.sub.2 O). The processing operations employed, usually flotation, leaching, evaporation and crystallization, are primarily intended to separate reasonably pure potassium salts from the associated sodium or magnesium salts found in these ores. Where highly purified potassium salts are required, the processing, especially to remove any sodium present, can become quite elaborate. For example, U.S. Pat. No. 2,715,608, issued to Casciani and Lang on Aug. 16, 1955, teaches the electrolysis of a purified brine derived from sylvinite in a diaphragm cell of the type used in conventional chlor-alkali production to produce a mixed alkali metal hydroxide solution. When concentrated and cooled, potassium ions combine with the chloride ions normally present in the catholyte product to form a precipitate of very pure KCl. In this process, KCl is the primary product of interest with the caustic soda, chlorine and hydrogen normally produced being considered as by-products.
Most recently, it has been found that the eutectic of about 40 wt.% KNO.sub.3.60 wt.% NaNO.sub.3 offers a number of particular advantages for use as a heat transfer medium in solar power systems. However, because of the relatively high processing costs encountered in converting KCl to pure KNO.sub.3, it has been impractical to produce large tonnages of reasonably pure mixed alkali metal nitrates. Thus, there is a need to be able to economically produce such a mixture starting from a natural sodium and potassium bearing ore such as sylvinite with a minimum of additional processing.